The present invention is directed to methods and apparatus to non-destructively read and play the audio signal of a standard vinyl phonograph record. Specifically, the present invention relates to audio systems which employ light beams and optics, rather than a mechanical stylus, to follow the spatial modulations of the record groove.
Conventional analog audio records store information in the grooves. The grooves are typically formed as left and right walls cut into a master disk with a mechanically vibrating stylus. The physical geometry of the left and right groove walls, known as modulations, carry the audio waveform as recorded information. Many conventional vinyl records are reproduced from the master disk.
A record is played with a device commonly known as a "pick-up." Typically, a stylus or needle is mounted in the pickup, and lowered into the groove, which acts as a guide for the stylus to follow. The stylus is in physical contact with the groove. As the record is rotated on a turntable, the changes in the right and left groove wall geometries, that is, the surface displacements, cause the stylus to vibrate. The stylus vibrations are then converted into electrical signals that are delivered to the input of an amplifier.
Conventional analog recording and reproduction systems have several inherent flaws. Imperfections in the recording medium and in the reproduction system can alter the playback waveform and distort the sound. Conventional reproduction systems are sensitive to "ticks" and "pops" caused by imperfections or dust particles in the grooves, surface scratches and low frequency noise caused by the system's physical vibration. In addition, since the stylus is in physical contact with the record groove, record wear (typically in the form of pits in the groove walls) is inevitable. Lastly, since conventional reproduction systems are mechanical, inertia affects the response characteristics.
In recent years, there has been research into and development of optical recording and reproduction systems. In general, digital reproduction systems use laser sources (coherent light) to detect the presence or absence of reflected light from recorded "pits" along a track. These digital systems require recording, processing and reproduction techniques very different from those for the conventional analog record. The use of light and optics to play conventional analog records has largely been ignored or found to be unsatisfactory.
A conventional analog (phonograph) record has a recorded mono signal (music, for example) in a groove where the groove is formed by at least a first wall having a position modulated by the recorded signal.
A conventional analog record has a recorded stereo signal encoded in the velocity of two orthogonal groove walls. The two walls form a "V" groove in the record with each wall forming an angle of approximately 45 degrees with the plane of the record.
The flat part between the grooves is called the land. The top edge of each wall is called the land/groove interface. Record parameter values that have been measured include:
______________________________________ Groove Width 0 to 150 um Groove excursion 0 to 150 um Land Width 0 to 300 um Wall Angles 41 to 49 degrees Modulation Angles -45 to 120 degrees Land Angles from -2 to 10 degrees Horizontal (warp) Groove Pitch 50 to 300 um/groove ______________________________________
These measured values extend far outside the standards set by the standards committees of the RIAA, AES, and NAB.
The modulation angle for a conventional record is the angle that the groove wall makes with respect to the tangential velocity vector. This angle is related to the desired signal by wall angle =atn{Vs/Vt } ##EQU1##
In addition to the problems caused by values outside the standard ranges, there are other record defects that occur in a large number of records. These defects result from cutting the groove wall at modulation levels higher than the cutting stylus can controllably cut and from carelessness in the cutting and mastering process.
When the groove is cut in the lacquer master with a stylus that is dull, oriented improperly or heated improperly, some of the material (chip) removed from the groove is deposited in chunks on the land/groove interface. These chunks of material are called "horns". With properly cut records, the material (chip) comes out of the groove in a fine string. The horns that do exist are removed from some records in an intermediate mastering step. For optical systems, these horns are irregularly shaped and tend to scatter incident light rather than uniformly reflecting incident light. The horns also make the location of the land/groove interface difficult to determine.
In theory the two groove walls are independent, representing two independent audio channels. The record industry, recognizing that, in general, the listener cannot hear stereo effects below 400 Hz, has added the left and right signals together to form a mono signal below 400 Hz. This mono signal is cut into the record as a constant groove width lateral motion.
Vertical modulation occurs when the left and right signals are 180 degrees out of phase and the groove width varies with the signal. The lateral groove excursions have been observed to exceed the groove width for signals up to 450 Hz. These excursions must be tracked by the tracking system to prevent the light beams from improperly sensing (playing) the land rather than properly sensing the groove.
The audio information signal is cut in the groove wall velocity so that the position waveform (the groove wall excursion) is proportional to 1/f where f is the information signal frequency. If this relationship were used directly, the 20 Hz signal-wall motion would move so much that the groove could not be cut and the excursion for a 20 KHz signal would be 1000 times smaller than for a 20 Hz signal and would, therefore, be lost in noise. To compensate for this large range, the RIAA (and the record industry) has accepted a standard RIAA equalization filter to reduce the levels at low frequencies and boost the levels at high frequencies before the cutting process. The inverse of the RIAA filter is used in playback to recreate the desired signal.
In spite of the RIAA filtering, however, the low frequency signals have much greater excursions than the high frequency signals so that the lower frequency signals need to be tracked more exactly than the higher frequency signals.
Various methods and systems have been proposed to play vinyl records using light and optical sensors to replace the conventional mechanical stylus and cartridge systems. Many of these systems require the light hitting the record to maintain its proper location with respect to the groove being played. Tracking techniques that have been used have produced tracking errors and audio distortion when playing many records.
The above-identified cross-referenced patent application discloses an optical turntable system for optically playing phonograph records without mechanically contacting the records and therefore without causing wear to the records. In that patent application, an optical unit includes an optical source providing a first light beam incident onto the wall to provide a reflected beam from the wall at a reflected angle proportional to the recorded signal. The optical unit also includes an optical sensor for sensing the reflected angle of the reflected beam to provide an output data signal proportional to the recorded signal. A drive assembly moves the record relative to the optical unit.
The optical unit is partitioned into a data extraction unit and a tracking assembly. The tracking assembly functions to position the optical unit over the section of the groove of interest as the record turns. The optical unit employs optical detectors so that contact with the record is not required either for data extraction or for tracking.
In an embodiment described in the cross-referenced patent application, the tracking assembly includes a vertical unit for positioning the optical unit a predetermined height from the record, includes a lateral unit for lateral positioning, and includes a tangential unit for tangential positioning. The lateral unit has a lateral detector for detecting the lateral position of the groove and for providing a lateral error signal as a function of the lateral displacement of the light beam relative to the groove. A lateral servo is responsive to the lateral error signal for tracking the light beam in the groove. The lateral detector includes a first detector for providing a first detector signal for indicating the lateral position of the first wall and includes a second detector for providing a second detector signal indicating the lateral position of the second wall. An electronic circuit is provided for processing the first detector signal and the second detector signal to provide the lateral error signal.
In the optical unit of the cross-referenced patent application for stereo operation, a first detector includes a first optical source to provide a first light beam incident onto the first wall which provides a first reflected beam to a first sensor. A second detector includes a second optical source to provide a second light beam incident onto the second wall which provides a second reflected beam to a second sensor.
In the stereo system, the incident laser beams are focused into small spots, one on each groove wall. The incident beams are directed in the plane normal to the tangential velocity vector at angles from approximately 50 to 70 degrees above the plane of the land which is the horizontal plane of the record. The reflected beams from the groove walls are reflected at angles of approximately double the groove-wall modulation angles. Each reflected beam (first or second) has a position measured by the centroid of the beam as the reflected beam impinges on a sensor. Each sensor is a position sensitive device (PSD) sensor that measures the position of the beam and produces an output signal proportional to that position. By processing the output signals from the PSD sensors, signals proportional to the audio signals cut into the groove walls are produced and also separate signals proportional to the total light power collected by the PSD sensors are produced.
The turntable system employs independent vertical (warp and vertical changes) and lateral (groove motion) tracking systems to keep the incident beam hitting the groove wall properly. The vertical tracking system reflects a laser beam off from the horizontal record surface (the land) and a sensor converts the motion of the reflected beam into a signal proportional to the distance of the record below the optical unit. A portion of the optical unit is moved vertically under servo control to keep the gap between the optical unit and the record at a constant height as the record moves.
In the cross-referenced patent application, the lateral tracking system uses the two data beams to produce the tracking information. The data beams are aligned so that the beams reflected off from the groove wall being played will be partially truncated by the top of the opposite groove wall as the reflected beam is reflected toward the PSD sensor. The two data beams are directed by a servo-controlled scanner mirror and thus are slaved together as they impinge on the record. If the beams are too far to the left, the right channel PSD sensor collects less light than the left channel PSD sensor, indicating a positive lateral tracking error. If the beams are too far to the right, the left PSD sensor collects less light than the right PSD sensor, indicating a negative lateral tracking error. The servo controlled scanner mirror is dynamically positioned to keep the intensity collected by the two PSD sensors equal to each other thereby providing the desired tracking.
While the lateral tracking system in the cross-referenced application is simple, it has a number of fundamental problems. First, the audio signal is derived from a beam that is partially truncated. This truncation tends to create some noise and distortion in the audio signal. Second, the truncation occurs on the land/groove interface, a region where horns, scratches, pits or other anomalies distort the truncating edge. These anomalies add to the distortion and noise in the audio signal. Third, the incident beams are forced deeper into the groove for wider groove widths. When the incident beam is deeper into the groove, a greater portion of the reflected beam does not reflect out of the groove and back to the PSD sensor due to increased truncation on the opposite groove wall. Fourth, the system actually tracks movements of the opposite groove wall through truncation on the opposite wall rather than tracking the wall from which the audio signal is being derived. Fifth, the tracking system does not provide information on the actual physical trajectory independently for each groove wall which is useful in order to correct some audio distortions.
In light of the above background, there is a need for a tracking system that corrects all of these problems and which provides an improved optical turntable system.